HTLV-I is thought to be directly involved in malignant transformation in adult T-cell leukemia and indirectly associated with other malignancies, myelopathies, neurologic disease (TSP/HAM), and chronic cutaneous infections. Studies are underway to examine the mechanism whereby the same virus appears to contribute to different types of diseases. Peripheral blood lymphocytes from HTLV-I seropositive individuals have been shown to undergo spontaneous proliferation when placed in culture. The levels of proliferation have shown some association with the diseased states and thus have been interpreted as reflecting viral activity in vivo. We have undertaken experiments to determine the nature of the lymphocyte population that is infected and proliferating and the mechanism whereby this response is induced. Lymphocytes from HTLV-I-infected and noninfected individuals were tested prior to and after 6 days of culture for cell-surface markers, cell activation, and virus and cytokine produc- tion. Cells were separated on the basis of their surface markers and examined for proviral sequence and virus expression. CD4+ lymphocytes expressing the activation markers, HLA-DR and CD25, increased marginally in cultured cells from infected individuals. In HTLV-I-infected cultures, 3H-thymidine incorporation increased and virus production was increased with and without the addition of mitogens and cytokines. Antibodies that react with cell surface molecules that regulate lymphocyte activation and antisera that identifies viral proteins were added to the cultures. CD4+ and not CD8+ cells contained and expressed virus. Antibodies to the alpha/beta and delta/gamma T-cell receptors specifically activated the T- cells from HTLV-I-infected individuals. Activation of cells could be reversed by the addition of antibodies to MHC class I antigens and beta2 microglobulin. A patient infected with HTLV-II was diagnosed as having large granulocytic leukemia by cell surface marker studies. The expanded cell population was isolated by flow cytometry as was CD4+ and CD8+ cell populations. DNA and RNA were prepared for determination of viral integration and expression.